Seasonal changes in learning ability have been described in various vertebrate species. Studies of changes in sexually dimorphic learning abilities and its neural basis in birds have yielded important new insights to our understanding of the vertebrate brain. Yet much of the significance of this work remains limited to the special case of song learning in birds. I propose a mammalian model for the seasonal modulation of cognitive sex differences: spatial learning ability in rodents. Recent work on several species of wild rodents, in particular the meadow vole, Microtus pennsylvanicus, has shown that sex differences in spatial learning are modulated by seasonal cues, such as day length. Moreover, there is preliminary evidence that the size of the brain structure underlying this behavior, the hippocampus, also changes seasonally. Seasonal changes in spatial learning have also been reported in humans, and as the hippocampus plays a key role in human memory, a rodent model of this phenomenon would have important theoretical and clinical implications. Until now, the further development of such a rodent model has been hampered by insufficient knowledge of natural spatial behavior in these species, particularly in the winter. In the past, understanding the function of natural space use in rodents has proved critical to predicting the patterns of learning and brain structure observed in the laboratory. The gap in our knowledge of winter behavior has been due to the technical difficulties of using radiotelemetry to track small mammals in this season. I propose here a novel use of an existing technology, passive integrated transponders, to solve this technical problem. Using an array of scanners to detect the movements of wild meadow voles tagged with transponders, I propose to document seasonal changes in space use. Although this novel technology could spawn numerous applications for the study of behavior, its present purpose would be to collect detailed movement data during the critical transition period between nonbreeding and breeding status in late winter and early spring. The array will be installed in a preexisting meadow vole study enclosure, so that the entire population can be marked and monitored. Finally, to relate sex differences in natural movements to changes in learning ability, voles from six density-matched control populations from similar study enclosures, will be shipped to my laboratory for measurement of spatial learning ability, using standard assays.